Referring to FIG. 1, conventional light-emitting diodes or “LEDs” include thin layers of semiconductor material of two opposite conductivity types, typically referred to as p-type layers 20 and n-type layers 22. The layers 20, 22 are typically disposed in a stack, one above the other, with one or more layers of n-type material in one part of the stack and one or more layers of p-type material at an opposite end of the stack. Each LED typically includes a p-n junction layer 24 provided between the p-type and n-type layers. The various layers of the stack are deposited on a substrate 26, such as a sapphire substrate. The substrate may be cut to form a plurality of LED packages, each package including one or more light-emitting diodes and a portion of the substrate.
In operation, electric current passing through the LED package is carried principally by electrons in the n-type layer 22 and by electron vacancies or “holes” in the p-type layer 20. The electrons and holes move in opposite directions toward junction layer 24, and recombine with one another at the junction. Energy released by the electron-hole recombination is emitted from the LED as light 28. As used herein, the term “light” includes visible light rays, as well as light rays in the infrared and ultraviolet wavelength ranges. The wavelength of the emitted light 28 depends on many factors, including the composition of the semiconductor materials and the structure of the junction 24.
FIG. 2 shows a typical LED package 10 including p-type and n-type semiconductor layers 20, 22 mounted atop substrate 26. The LED is surrounded by a substantially transparent encapsulant 30. Each layer of the package has its own unique index of refraction. As used herein, the term “refraction” means the optical phenomenon whereby light entering a transparent medium has its direction of travel altered. In FIG. 2, LED 18 has an index of refraction designated n1, the transparent substrate 26 has an index of refraction designated n2 and the encapsulant layer 30 has an index of refraction designated n3. Because the index of refraction of the substantially transparent substrate 26 n2 is greater than the index of refraction of the transparent encapsulant 30 n3, many of the light rays generated by LED 18 are internally reflected back into the package and are not extracted therefrom. This is due to the optical phenomenon known as total internal reflection, whereby light incident upon a medium having a lesser index of refraction (e.g. encapsulant layer 30) bends away from the normal so that the exit angle of the light is greater than the incident angle θi. As θi increases, the exit angle approaches 90° for a critical incident angle θc, calculated using Snell's Law. For light rays having incident angles θi greater than the critical angle θc, the light ray will be subject to total internal reflection. As shown in FIG. 2, the incident angle θi for light ray 32 is greater than θc. As a result, light ray 32 is totally internally reflected within package 10.
Thus, in many LED packages the light rays generated by the LED are never extracted from the chip because such light rays are totally internally reflected within the package. Thus, there is a need for improved LED chips that optimize the amount of light that may be extracted from the packages. There is also a need for methods of making such chips.